Supporting Information. 3 Shaw Environmental Inc., Cincinnati, OH

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1 SI-1 The Impact of Environmental Conditions (ph, Ionic Strength and Electrolyte Type) on the Surface Charge and Aggregation of Silver Nanoparticles Suspensions Supporting Information Amro M. El Badawy 1, Todd P. Luxton 2, Rendahandi G. Silva 3, Kirk G. Schekel 2, Makram T. Suidan 1 and Thabet M. Tolaymat 2* 1 Dept. of Civil & Environmental Engineering, University of Cincinnati, Cincinnati, OH 2 US Environmental Protection Agency, Office of Research and Development, Cincinnati, OH 3 Shaw Environmental Inc., Cincinnati, OH * Corresponding Author: USEPA Office of Research and Development, National Risk Management Laboratory, 26 West Martin Luther King Drive, Cincinnati, OH 45224, tolaymat.thabet@epa.gov, phone: , fax: Submitted to Environmental Science and Technology Preparation Date 1/5/21 8 Pages 2 Tables 4 Figures

2 SI-2 1. AgNPs Synthesis Procedures Hydrogen reduced AgNPs (H 2 -AgNPs): Three grams of Ag2O (99+ %, Acros Organics) were added to 3 L of Milli-Q water in a 5 L Pyrex round-bottomed flask. The solution was heated on a hot plate and was maintained at 7 C. Once the temperature was stabilized, 48 L of ultra pure hydrogen gas per L of solution was bubbled through the solution at a standard pressure. Citrate reduced AgNPs (citrate-agnps): Briefly, a solution of 1x1-3 M AgNO 3 (99.99%, Aldrich) was mixed with a solution of 1x1-2 M Na 3 C 6 H 5 O 7.2H 2 O (99+ %, SAFC supply solution) in a volume ratio of 2:1, respectively. Instead of boiling, the mixture was heated at 7 C for 4 hours in a water bath. Borohydride reduced AgNPs (NaBH 4 -AgNPs): A solution of 5x1-3 M AgNO 3 was added dropwise (~1 drop per second) to a vigorously stirred ice-cold 2x1-3 M NaBH 4 solution at a final volume ratio of 1:3, respectively. Polyvinylpyrrolidone stabilized AgNPs (PVP-AgNPs): Commercial AgNPs powder with a particle size of 3 nm as per the manufacturer (the size was verified later using TEM (Figure S-2)) was purchased from Nanostructures and Amorphous Materials (Houston, TX, USA). The PVP-AgNPs suspension was prepared as follows: The PVP was dissolved in Milli-Q water and mixed well to achieve.25% solution. The commercial particles were weighted out and mixed with the PVP solution. The mixture was sonicated using ultra sonic dismembrator 55 for 2 minutes. The salt was added to achieve the required ionic strength. Then the ph was adjusted and the required measurement and analysis was performed for HDD and ζ potential. The stability of the HDD of the sonicated suspension was tested over 8 hours time period. Minimal change to the HDD was observed. Branched polyethyleneimine stabilized AgNPs (BPEI-AgNPs): Branched polyethyleneimine (BPEI) (99 %, Mw = 1.2 kg mol-1) and AgNO 3 were separately dissolved in 1x1-4 M solution of N-(2-hydroxyethyl) piperazine-n -2-ethanesulfonic acid (HEPES). The two solutions were then mixed in a volume ratio of 1:1 to give a final molar ratio of.5:1:.1 BPEI: AgNO 3 : HEPES, respectively. After mixing, the solution was exposed for 2 hours to UV irradiation using a standard low-pressure mercury arc lamp.

3 SI-3 2. Calculations for the decay in surface potential as a function of distance from the particle surface as influenced by the initial surface charge density. Derivations of the Gouy Chapman Diffuse double layer theory form first principals are presented in details by Gast (1). The following equations relating surface charge density (mol m -2 ) to surface potential (mv) and the potential (Ψ X ) at a distance X from the surface were derived by Gast (1). Surface charge as a function of surface potential: σ = 2 1/ 2 nεkt 2π zeψ sinh 2kT Rearrangement provides: Ψ 1 σ sinh 1/ 2 2εkT π = ze 2kT Potential at a distance X from the surface as a function of surface potential: Rearrangement provides: zeψ 4kT = zeψ 4kT Χ tanh tanh exp κχ Ψ Χ 1 ze X 4kT Ψ κ = tanh tanh exp 4kT ze Where σ = surface charge density (mol m -2 ), n = electrolyte ion concentration (mol m -3 ), ε = the permittivity of free space, k = the Bolzman constant, T equals temperature (K), e = the charge of an electron (C), z = charge of the symmetrical electrolyte, Ψ = surface potential (V), Ψ X = potential at a distance X from the surface (V) κ = the inverse of the double layer thickness, and X distance from the surface (m).

4 SI-4 TABLE S1. Standard errors for the hydrodynamic diameter of selected samples showing low variability NaNO 3 NaNO 3 NaCl Ca(NO 3 ) 2 Particle (1mM) (1mM) (1mM) (1mM) Average Zeta Potential (ζ) (mv) and Standard Deviation (mv) Size Stdev. Size Stdev. Size Stdev. Size Stdev. Citrate-AgNPs H 2 -AgNPs NaBH 4 -AgNPs NA NA PVP-AgNPs BPEI-AgNPs TABLE S2. Standard error for the zeta potential of selected samples showing low variability NaNO 3 NaNO 3 NaCl Ca(NO 3 ) 2 Particle (1mM) (1mM) (1mM) (1mM) Average Hydrodynamic Diameter HDD (nm) and Standard Deviation (nm) HDD Stdev. HDD Stdev. HDD Stdev. Size Stdev. Citrate-AgNPs H 2 -AgNPs NaBH 4 -AgNPs NA NA PVP-AgNPs BPEI-AgNPs

5 SI-5 3. Absorbance Citrate-AgNPs H 2 -AgNPs NaBH 4 -AgNPs PVP-AgNPs BPEI-AgNPs Wavelength (nm) Figure S1 UV-VIS spectra for the As-prepared AgNPs suspensions

6 SI-6 a) H2-AgNPs d) Commercial -AgNPs no PVP 55 nnm m b) Citrate-AgNPs e) PVP-AgNPs 55 nnm m c) NaBH4-AgNPs f) BPEI-AgNPs Figure S2 TEM image for the as prepared AgNPs a) H2-AgNPs b) Citrate-AgNPs c) NaBH4-AgNPs d) Commercial AgNPs without PVP e) PVP-AgNPs f) BPEI-AgNPs

7 Figure S3 Decay in surface potential as a function of distance from the particle surface as influenced by the initial surface charge density. The inset shows a more detailed depiction of the area within the highlighted box. All calculations were based on materials provided by Gast 26 and the equations are presented in supplementary information SI-7

8 SI-8 Figure S4 XRD Patterns for the H 2 -AgNPs with NaCl as a background electrolyte References 1. Gast, R. G. Surface and Colloid Chemistry. In: Dixon, J. B.; Weed, S. B. (Eds). Minerals in Soil Environments. Soil Science Society of America, Madison, WI, 1977,